This invention relates generally to nonwoven fabrics including nonwoven webs and nonwoven laminates, and more particularly concerns a durable nonwoven fabric.
Nonwoven fabrics have found use in industry as protective work garments. Such work garments are worn to protect a worker's clothing from dirt and grease as well as to protect the worker's clothing and the worker from splashing liquids or sprays which would harm either the worker's clothing or the worker.
Nonwoven fabric work garments at the present time are constructed from a nonwoven spunbond/meltblown/spunbond laminate (SMS) comprising a meltblown layer formed from thermoplastic microfibers which layer is sandwiched between two layers of spunbond fabric formed from thermoplastic filaments. Nonwoven spunbond/meltblown fabric laminates are disclosed in Brock et al U.S. Pat. No. 4,041,203 which is assigned to Kimberly-Clark Corporation, the assignee of the present invention. The SMS fabric laminate of current work garments is made from polypropylene and may either be treated or untreated to provide good repellency against light splash and spray of commonly encountered industrial low surface tension liquids. Such polypropylene SMS work garments are sold under the mark KLEENGUARD.RTM. by Kimberly-Clark Corporation, the assignee of the present invention. Such conventional polypropylene SMS work garments generally are only sufficiently durable to withstand a limited number of uses before they tear or pill.
The prior art discloses the addition of a nucleating agent to polypropylene to make higher tenacity polypropylene fibers. Wakatsuki et al., U.S. Pat. No. 5,039,748, discloses a high strength polypropylene fiber obtained by adding 0.05 to 10,000 parts per million by weight of vinylcycloalkane polymer to polypropylene having a molecular weight such that the intrinsic viscosity is from 0.5 to 2.5 dl/g as measured in a tetralin solution at 135.degree. C. Particularly, the molecular weight of the polypropylene has a ratio of weight average molecular weight (Mw) to number average molecular weight (Wn) of not more than 3.5/1. The vinylcycloalkane polymer is used as a nucleating agent to strengthen the polypropylene fiber.
Asahi Chemical Ind., Japanese Patent No. 61,155,437 discloses the addition of nucleating agents, such as alumina and silica of grain size below 5 microns, sodium, potassium, aluminum salt of adipic acid, dibenzylidene sorbitol to polypropylene having a high melt index. The polypropylene has a melt index of from 70-500 g/10 rain at 230.degree. C. The composition is used to produce polypropylene continuous filament nonwoven fabric which is said to have excellent uniformity of weight distribution of filaments. The amount of nucleating agent added to the polypropylene is from 0.05 to 0.5% by weight.
Kawai, U.S. Pat. No. 4,314,039 discloses the addition of 0.005 to 8 parts by weight of 1.3, 2.4-di(alkylbenzylidene) sorbitol with each alkyl group having 2 to 18 carbon atoms to 100 parts by weight of polypropylene. The polypropylene composition is useful in the production of molded articles having excellent transparency.
The prior art, however, does not disclose a nonwoven polypropylene fabric laminate, such as SMS, produced with a nucleating agent which fabric laminate is more durable than a conventional polypropylene SMS fabric laminate. Although the Wakatsuki et al. patent teaches an increase in the tenacity of fibers by adding nucleating agents, it does not necessarily follow, nor is it necessarily obvious, that the use of nucleating agents will give a stronger thermal-bonded spunbond web or SMS fabric laminate. In thermal-bonded continuous filament webs and in thermal-bonded fabric laminates constructed from such webs, such as thermally bonded SMS fabric laminates, studies have shown that the failure mechanism is one of brittle fracture of the fibers at the periphery of the thermal bond points. This suggests that the full strength of the fibers themselves is never completely realized in thermal-bonded webs and fabric laminates because a weakness is introduced by the thermal-bonding process at the bond points. It follows therefore that increasing fiber tenacity will not necessarily lead to a stronger thermal-bonded web or fabric laminate because the limiting factor is still the brittle bond points in the web or fabric laminate.
Consequently, there is a need for a nonwoven web and fabric laminate which has improved bonding characteristics and as a result, improved durability. Particularly, there is a need for a more durable nonwoven thermal-bonded spunbond web to be used as the outer layer of a more durable thermal-bonded fabric laminate.